The present invention relates generally to a control system for an electric vehicle, which performs independent torque control for traction motors for respective left and right wheels. More specifically, the invention relates to an electric vehicle control system which can provide enhanced vehicular driving performance.
Conventionally, electric vehicle control systems have been provided for independently driving left and right vehicular wheels with a plurality of driving motors, as shown in Japanese Unexamined Patent Publications (Kokai) Nos. JP-A-62-89403, JP-A-64-133804, JP-A-1-298903. In those control systems, there are two typical control methods. Namely, the control method for the electric motor disclosed in Japanese Unexamined Patent Publication No. JP-A-63-133804 derives motor speed commands for respective left and right driving motors on the basis of an accelerator pedal depression amount, a brake pedal force, a steering angular position and so forth, to perform speed control for the driving motors. This method is characterized by smooth cornering since optimal motor speed can be obtained relative to the steering angular position, as long as the road friction coefficient is constant.
On the other hand, the control method disclosed in Japanese Unexamined Patent Publication JP- A-62-89402 varies inverter frequency depending upon a torque difference so as to electrically achieve an equivalent system to the mechanical differential gear device. Unexamined Patent Publication No. JP-A-1-298903 discloses an electric vehicle control method in which motor torque commands for the left and right traction motors are derived in accordance with the accelerator depression amount, the brake pedal force, the steering angular position and so forth as parameters, and control is performed for the output torques of the traction motors based on the motor torque commands. The later two systems are characterized in that they generally have no speed control loop. With these methods, since it can be easily realized to have a function equivalent to the differential gear device which is normally employed for the driving system employing a single motor or engine for driving both the left and right wheels, the capability of driving the vehicle in accordance with the driver's desired direction even when the road friction coefficients are different at the left and right wheels is achievable.
However, the prior art disclosed in the publications set forth above have the following problems. According to, the former method of performing speed control, when the road/tire traction at the left and right wheels are different due to a difference of the road friction coefficients or a difference of the tire pressures, the vehicle may not be driven in a straight line at the neutral steering angular position. In the second method, since the torque is controlled solely by the inverter frequency, it can perform torque control only under stead state conditions. At the transition state, this method may not perform torque control in the event of a torque difference in the transition condition during the motor driving operation, straight line stability and cornering performance of the vehicle can be degraded. Furthermore, the third method for performing torque control can cause a substantial wheel speed difference when the road friction coefficients at the left and right wheels are significantly different, such as on a muddy road, a snow covered road and so forth. Such substantial difference of the wheel speeds can lead the vehicle into spinning when the road friction coefficient is abruptly increased.